1. Field of the Invention
The present invention relates generally to a communications system and, more specifically, to optimizing the capacity of a satellite-based communications system.
2. Description of the Prior Art
Conventional satellite-based communication systems maintain an orbit above the earth and contain at least one antenna that provides coverage to an area on the earth's surface by producing a series of beams that divide the antenna's coverage area into a pattern of contiguous circular regions or cells. Operationally, an antenna beam dwells in each cell in a fixed sequential pattern to nominally cover the satellite coverage area where there are multiple users (receivers) distributed across the cells. A particular concern in conventional satellite-based systems is the efficient use of the communication spectrum allocated to a particular satellite service. Generally, the users within a satellite's coverage area are non-uniformly distributed and therefore, the coverage area or visible area of a particular antenna may contain one or more cells that are densely populated and surrounding cells that are less densely populated. Specifically, at any time, an antenna beam may service a cell where the density of users within the cell is lower than the allocated spectrum capacity and therefore, the cell's spectrum capacity is not efficiently used.
In conventional systems where time division multiple access (TDMA) protocol are used, there may be, for example, one-hundred time slots in a time frame for a six-cell coverage area. The antenna beam dwells within each cell on a frame by frame basis making available each of the one hundred time slots to a particular cell. For those cells where the user demand does not meet the one hundred time slot allocation, the remaining time slots will be quiescent. Likewise, in systems where frequency division multiple access (FDMA) protocol are used, access slots are assigned by frequency bands and the satellite's available spectrum is divided into frequency channels one through N. Using the previous example, a conventional FDMA system antenna beam may dwell within each cell of the six-cell coverage area on a frame by frame basis, making available all N frequency slots to users within a particular cell. Similar to the TDMA example, allocated communication spectrum is wasted when frequency slots are not used because of low user density within a particular cell region.
Although satellite-based communication systems, similar to those described in the previous examples, provide some level of service to all users contained within a coverage area, they are inefficient and do not optimize coverage for those cell regions which have long term high capacity demands. These inefficiencies can cause non-optimal use of communications spectrum capacity during those time periods where low-capacity areas are being serviced.
Based on techniques known in the art for satellite-based communications systems, a communications system that allows increased simultaneous coverage of users and provides higher gain when specifically covering users in high capacity areas is highly desirable.